1. Field of the Invention
The present invention relates generally to a device and the method to synchronize timers. More specifically, the invention relates to the device and the method to synchronize a master oscillator located at a base station and a slave oscillator located at a microcell. Such synchronization facilitates simulcasting of radio frequency channels.
2. Information Disclosure Statement
A method of synchronizing clocks is taught in the prior art in J. Levine et al., "The NIST Automated Computer Time Service," Journal of Research of the National Institute of Standards and Technology Vol. 94, Number 5, p. 311 (September-October 1989). In that article, the authors explain the Automatic Computer Time Service (ACTS) which is a telephone time service designed to provide users with telephone access to time generated by the National Institute of Standards and Technology (NIST).
The article cites the principle limitation of time dissemination as being the uncertainty in the velocity of the propagation of the information through the medium separating the transmitter and the receiver. The delay, typically in the order of milliseconds, depends upon the physical length of the path and on the group velocity of the signal. Generally, neither of these parameters is known. Consequently, to achieve the highest degree of synchronization, real time measurements of the transit time must be taken.
The delay can be determined by measuring the time duration between transmitting a pulse to the user and receiving an echoed pulse. Half of this round trip delay is the time required for the signal to reach the user assuming that the communication medium is reciprocal. Although the article states that empirical evidence indicates reciprocity in telephone paths, it also maintains that the lack of perfect reciprocity may prove to be the ultimate limitation in the system.
To achieve this millisecond synchronization of computers, the NIST has developed a simple telephone system for the automated setting of clocks in digital systems. NIST has developed software to facilitate automatic dialing, selecting a time zone, selecting a mode of operation, echoing the on-time marker (OTM), setting of the computer clock, archiving of clock off-set, and transmitting a signal to the parallel port of a computer, which can be used to produce an external time pulse coincident with the OTM. Additionally, the article provides diagrams of simple circuits to convert the signal sent to the printer port to a positive pulse. The article also details a circuit that echoes all characters from the user and provides an external pulse when the OTM is received. This circuit requires a modem but does not require a computer.
The article basically focuses on the application of ACTS for the maintenance of accurate time within a digital computer. For example, the article suggests tagging business transactions or scientific data with the exact time. Such tagging becomes particularly useful where a multiplicity of computers independently tag events, and thus require a common time standard. Additionally, the article mentions a second application aimed at the measurement of frequency. In this application, the pulses coincident with the OTM are used to start and stop a counter. The counter counts the output of the oscillator under the test. By comparing the pulses to the counts, an oscillator can be calibrated using exact time. The system can be used to set digital clocks, or to perform frequency calibrations at an accuracy of one part in 10.sup.8 for a one day period.
The article also speculates on the future development of the system and the new products which it might spawn. It suggests a digital clock comprising a microprocessor incorporated into a phone receiver. To maintain exact time according to the National Institute of Standards and Technology, the clock would access the ACTS via the telephone. Such a phone could be networked to other phones in a school or factory to provide the exact time throughout the organization. Additionally, the article suggests automatic calling to enable the user to call during less expensive non-peak hours.
Thus, the article limits the application of the method of synchronization to time--both in the maintenance of accurate clocks and the calibration of frequency oscillators. The prior art lacks motivation to use the synchronization method in telecommunications. More specifically, the article does not suggest that the ACTS could be modified to enable a base station and a microcell to simulcast.